338 Lapua Magnum: Barrel Length Versus Muzzle Velocity (30-17 inches)

One of the decisions we make when we build or buy a rifle is how long the barrel should be. Like everything else in life, barrel length is a trade off. The longer barrel comes with the advantage of higher velocity and increased downrange performance at the price of mobility and weight. The shorter guns are quicker to handle and often easier to tune, but have lower muzzle velocities and a louder muzzle report. For the one-sport shooter, barrel length maybe an easy decision based on the rules of the game, however, for the rest of us it requires thought.

In this post, we’ll take a look at how barrel length affects muzzle velocity in a 338 Lapua Magnum. The 338 Lapua is a “newer” cartridge and designed from the ground up for military usage. Build on a modified 416 Rigby case, the Lapua dispenses with the head spacing belt traditionally associated with magnum cartridges for a rimless case.

In practice, the 338 Lapua Magnum effectively bridges the gap in performance between the 7.62x51mm NATO and the 50BMG. Offering significantly increased range and terminal performance over the 7.62- without the size, bulk and associated recoil of the 50.

The purpose of this post is to examine how barrel length affects muzzle velocity in the 338 Lapua Magnum. Since empirical data sets provide a reference point, we’ve settled on cutting barrels under controlled conditions for this type of experiment. This way we can shoot the same ammunition from the same barrel, in the same chamber, with the same action at the same ambient temperature. This type of testing requires a test gun with a donor barrel (which is destroyed during the process), which can get quite expensive.

Recently, Brandon, one of the moderators from Precision Addiction offered to send us his 338 barrel for use in a test on the 338 Lapua Magnum. I took him up on his offer and he sent me his used Pac-Nor chrome moly barrel with about 600 rounds though it. This thing was a beast! A heavy 1.350″ shank that ran straight for 6″, until tapering to 1″ at 30″ in length! I just needed to put it on a rifle.

This is the test gun I came up with. It was built with the following parts from Brownells:

The test barrel had been previously chambered on a Surgeon action, which has a larger barrel tenon. I simply dialed the barrel in on the lathe, turned down the tenon and threaded it for the Defiance action. Instant 338 Lapua Magnum rifle with 600 plus rounds down the tube. The barrel was headspaced with Manson gauges.

I used the DRO on the lathe to lay out cut lines along the barrel in 1″ increments. This would provide a visual reference for cuts made with the saw.

WARNING: The loads shown are for informational purposes only. They are only safe in the rifle shown and may not be safe in yours. Consult appropriate load manuals prior to developing your own handloads. Rifleshooter.com and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.

Due to budget constraints, I decided to shoot 4 rounds of each load at a given barrel length; record the velocity with a MagnetoSpeed barrel mounted ballistic chronograph, cut the barrel back an inch, and repeat test. This would give us 4 shots worth of data from each cartridge at each barrel length from 30″ down to 17″ and expend 112 rounds of 338 Lapua ammunition.

I’ll be honest, throwing all those heavy bullets down range in an afternoon on a rifle without a brake wasn’t exactly something to look forward too, but, hey, someone has to do it!

The results of my testing are shown below.

You’ll notice the 250 SMK loads dropped fairly consistently. The 300 SMK load showed a slight increase from 30 to 29″. I’ve recorded this in other tests and it seems to be more common with a heavier load. I suspect it is primarily due to the small sample sizes being used along with the relative proximity of muzzle velocities in adjacent lengths. Had I doubled the sample size I suspect this may have looked a little different. Had I proceeded in 2″ increments, this increase would not appear in the data.

I’ve further dis-aggregated the data set by load. Each of the following tables shows the barrel length, muzzle velocity (MV), standard deviation (SD), change in velocity from the previous barrel length (CHG), change in velocity from the initial barrel length (CHG 30″) and the rate of change which is calculated by dividing the total change in velocity by the number of inches the barrel has been reduced.

The 250 grain 338 Lapua Magnum load had a maximum velocity of 2,942 feet/second with a minimum velocity of 2,547 feet/second, for a loss of 395 feet/second.

For the 300 grain SMK, velocities ranged from 2,871 feet/second (29″) to 2,492 feet/second for a total velocity loss of 341 feet/second.

To help facilitate discussion about how barrel length affects velocity, I plotted the rate of change for both cartridges on the same set of axes.

Note that after the initial rate change, the rate of the change in velocity is fairly consistent. Note: this isn’t showing velocity, so a horizontal line would show the velocity is decreasing and the rate of change is constant. The downward trajectory of the slopes in these lines show that as you decrease barrel length, the rate at which both rounds lose velocity increase.

So how do these results affect our ability to hit targets downrange?

I modeled each load at each barrel length in my ballistic calculator using G7 ballistic coefficients from Litz. These numbers assume the shooter is at sea level, in 59F, with a scope 1.75″ above the bore and a target distance of 1,000 yards. Results are displayed in MRAD.

I like using MRAD for these discussions, since .1 MRAD is common to most scopes and represents a coarseness in adjustment (approximately 3.6″ at 1,000 yards) that I think more readily translates into real world use rather than measuring the drop in another unit, like inches.

First, a look at how barrel length affects drop

You’ll note at barrel lengths above 26″, both bullets don’t give up a lot of elevation at 1,000 yards. However, as barrel length decreases, especially below 22″, the change in trajectory becomes more significant.

Readers always ask about this one for some reason. I graphed the standard deviation of each load on a set of axes, here it is (above). I think the normal distribution is the most abused probability distribution function in the world. While I think it gives you a glimpse into your hand loading, it doesn’t tell the whole story with small sample sizes. For this reason, I present the graph above with a little reservation. If those were 30 shot groups, I could see it having more value.

I would say anything about 26″. If the length isn’t an issue, you may be able to work slower powders into higher velocities at the longer lengths. While the 17″ barrel still performed better than some other options, it didn’t allow the cartridge to reach its full potential. I don’t know of anyone with a 20″ 338 Lapua Magnum that is happy that they went short.

Once you get into the big 338s, you have some other cartridges that are very similar. The 338 Lapua has a case capacity of 114.2 gr of water, while the 340 Weatherby- 98.0 gr, 338 Remington Ultra Mag- 113, and the capacity of 338-378 Weatherby is 125 gr. I’ve sent hundreds of rounds of 340 Weatherby down range and was enamored with the cartridge in the late 90s.

340 Weatherby and 338 Lapua Magnum

I don’t understand why the 340 Weatherby (above, left) isn’t compared more to the 338 Lapua Magnum (above, right), especially with the 250 grain class bullets. True, the 338 Lapua holds around 15 grains more water, but they both push a 250 grain bullet at similar speeds. Factory specs on a 250 grain bullet from a 26″ 338 Lapua are 2,950 feet/second. My 340 Weatherby shot a 250 grain bullet 2,948 feet/second all day long. When the Lapua started gaining traction in the US, I kept asking guys why they just didn’t shoot a 340.

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Designed to bridge the gap between 308 Winchester/7.62 NATO and 50 BMG, the 338 Lapua Magnum (8.6×70) has developed an impressive following with long range shooters. When I built mine, I settled on a 22″ barrel, […]